I also encountered an instance in 1987 where 3-wire electric dryer and stove outlets had 240 volts corner grounded in them and the system was C-phase grounded rather than B-phase grounded. This created a sparking surprise when a tenant for a temporary factory got some 3 and 5 KW protable electric heaters and used the center wire of dryer and range cords as the equipment "ground". That is, the "neutral-ground" of the receptacle was 240 volts to ground. I had to change the wiring of these receptacles to put the neutral on the center terminal.

I tell people that electrical equipment contains surprises. Thanks to snakes and yellow jackets electrical equipment does not need to even be energized. There are some photos over at www,themeterguy.com where snakes have popped the cover off of meter sockets and wrapped themselves around the 240 volts or 480 volts. Since snakes do not have perspiration glands they can do this to warm themselves up without geting electrocuted. There is even a picture of 2 snakes using a meter socket as the Rock Em Sock Em Motel!

Akron, Ohio has 240 volts corner ground systems, both utility supplied and separately derived, where sometimes the B-phase is grounded or the C-phase is grounded. How we get C-phase grounded is that the neutral bar of some fusible switches is on the right and the color code is black red blue. Since blue is neutral in a number of European countries it does make sense. In the beginning of 1995 I told somebody from IBEW to think of it as a British corner ground.

How fusible fusible switches should be wired on corner ground is to put the grounded phase in the center switch and fuseholder of a genuine 3-phase switch. This give you the option of installing a dummy fuse or providing overload protection for the grounded phase. Article 430 used to require a fuse of circuit breaker pole in the grounded conductor of a corner grounded motor circuit. Perhaps this was taken out because corner grounded is obsolete but I know that it was still in there in 1993.

When installing circuit breaker panelboards on 240 volts corner grounded I have always insisted on using a panel with 3 phase busbars so that there is a circuit breaker pole in the grounded conductor of a branch circuit. The grounded conductor of a corner grounded system is NOT a neutral and is subject to overloading from unbalanced single phase loading. Also, a genuine 3 phase panelboard makes it possible to hook up a 3-wire T primary 4-wire T secondary transformer that supplies 120/240 4-wire delta. A better way to get 4-wire delta is to use 2 single phase transformer in the T primary T secondary configuration. A 3-wire floating neutral wye primary 4-wire delta secondary transformer like in utility practice needs 5 supplementary overload relays, one in each primary hot wire and one for each half of the center tapped secondary winding. T primary T secondary can use convenbtional primary and secondary protection and the impedance imbalance is a lot less than open delta vee. Also, this configuration avoids currents that might circulate in a delta winding.

Maybe that was off the subject, but opening a fuse or circuit breaker pole in the grounded conductor of a corner grounded system only causes single phasing, not the overvoltages that occur when a 120/240 volt neutral opens.

Corner grounding of 240 volts is better than ungrounded operation opartially since ungrounded systems can build up static electricity during rainstorms if a ground detector is somehow disconnected or never installed. Sure, 240 volts corner grounded puts as much stress on circuit breakers as 240Y416 or 277Y480. Some manufactures are actually pushing the use of 277Y480 volt circuit breakers on 240 volts corner grounded so as to avoid misapplication of 120/240 volt circuit breakers and to get improved short corcuit ratings.

Mike Cole, mc5w at earthlink dot net